The present invention relates to an optical detector, an optical pickup, and an optical information reproducing apparatus (hereinafter described as an optical disc drive) using an optical pickup, capable of reproducing an information signal recorded in an optical information recording medium (hereinafter described as an optical disc).
There are various types of optical discs having different substrate thicknesses and using different wavelengths. For example, discs such as CD and CD-R have a substrate thickness of 1.2 mm and use a 780 nm band as an optimum recording/reproducing laser wavelength, whereas recently standardized discs such as DVD-ROM and DVD-RAM have a substrate thickness of 0.6 mm and use a 650 nm band as an optimum recording/reproducing laser wavelength. Optical disc drives using a laser beam having a wavelength shorter than conventionally used wavelengths have been proposed as well. Under such circumstances, the main trend of, for example, optical pickups for DVD prevailing nowadays, is to mount semiconductor lasers having two different wavelengths in 780 nm and 650 nm bands in order to be compatible with CD optical discs already widely used.
Wide use of such optical discs is forcing optical disc drives to be compact and inexpensive. Techniques of making optical pickups compact and simple are therefore essential. Effective means for compact and simple optical pickups are to reduce the number of components of an optical system, to configure an optical pickup with inexpensive components, or the like. If an optical disc drive is to be made compatible with a plurality type of optical discs, an optical system characteristic to each type of the optical disc is required. In this case, simplifying an optical system and reducing the number of components of an optical system by using common optical elements is effective for making an optical pickup compact and inexpensive. JP-A-8-55363 and JP-A-9-54977 disclose the technique of guiding each of the laser beams of two semiconductor lasers into the same intermediate optical path to reproduce information recorded in a plurality type of optical discs with one objective lens.
Most of optical pickups each having two semiconductor lasers have an optical system whose converging optical elements such as an objective lens and a collimator lens are used in common in order to make an optical pickup compact and inexpensive. An example of the structure of such an optical system is shown in FIGS. 1A and 1B.
Referring to FIG. 1A, a light beam emitted from a semiconductor laser 11 oscillating at a wavelength of, for example, 650 nm, reaches a dichromatic half prism 12. The dichromatic half prism 12 is an optical element made of two prisms adhered each other, and has therein a reflection film which reflects a laser beam of 650 nm in wavelength by about 50% and transmits it by about 50%, and transmits a laser beam of 780 nm in wavelength by about 100%. The light beam emitted from the semiconductor laser 11 is reflected by the reflection film of the dichromatic half mirror 12 disposed at an angle of 45xc2x0 relative to the optical axis of the semiconductor laser 11, then converted into a parallel light beam by a collimator lens 5, and reaches an objective lens 6. The objective lens 6 is integrally held by an actuator 7. By energizing a drive coil 8, the light beam can be focussed to form a light spot on a data record surface of an optical disc 1 such as DVD-ROM. A light beam reflected from the optical disc 1 propagates along a reversed optical path of the incoming optical path to reach the dichromatic half prism 12 via the objective lens 6 and collimator lens 5. About 50% of the reflected light amount transmits through the dichromatic half prism 12 and reaches a dichromatic half mirror 13. The dichromatic half mirror 13 is an optical element which transmits a laser beam of 650 nm by about 100%, and transmits a laser beam of 780 nm in wavelength by about 50% and reflects it by about 50%.
The light beam reached the dichromatic half mirror 13 transmits therethrough and thereafter is converged at a predetermined position of an optical detector 14.
Referring to FIG. 1B, a light beam emitted from a semiconductor laser 15 oscillating at a wavelength of, for example, 780 nm, transmits through a diffraction grating 16 for generating three beams and thereafter reaches the dichromatic half mirror 13 disposed at an angle of about 45xc2x0 relative to the optical axis of the semiconductor laser 15. As described above, the dichromatic half mirror 12 has the characteristics that a laser beam having a 780 nm wavelength is reflected by about 50%, and the dichromatic half prism 12 has the characteristics that a laser beam having a 780 nm wavelength is transmitted by about 100%. Therefore, the light beam emitted from the semiconductor laser 15 is reflected by the dichromatic half mirror 13, transmitted through the dichromatic half prism 12, thereafter converted into a parallel light beam by the collimator lens 5, and reaches the objective lens. The objective lens 6 can focus the light beam emitted from the semiconductor laser 15 to form a light spot on a data record surface of another optical disc 10 such as CD-ROM. A light beam reflected from the optical disc 10 propagates along a reversed optical path of the incoming optical path to reach the dichromatic half mirror 13 via the objective lens 6, collimator lens 5 and dichromatic half prism 12.
Since the dichromatic half mirror 13 is an optical element transmitting about 50% of the laser beam having a 780 nm wavelength as described earlier, the light beam reached the dichromatic half mirror 13 transmits through the dichromatic half mirror 13 and thereafter is converged at a predetermined position of the optical detector 14.
In the structure of the optical system illustratively shown in FIGS. 1A and 1B, the converging optical system from the dichromatic half prism 12 to the collimator lens 5 and to the objective lens 6 is used in common so that the number of components is reduced. The dichromatic half prism and dichromatic half mirror having a selection function of two different wavelengths and predetermined optical characteristics are used, and the semiconductor lasers having two different wavelengths are used. These optical elements having the waveform selection function and the semiconductor lasers are much expensive than other components of the optical pickup, and hinder a further cost reduction of the optical pickup.
Under such circumstances, an object of the present invention is to provide an optical detector, an optical pickup, and an optical disc drive using an optical pickup capable of recording/reproducing an information signal in/from a plurality type of optical discs, in which the structure of an optical system simpler than that of a conventional optical system can be realized and a low cost optical system structure can be realized by using inexpensive optical elements and semiconductor lasers as small in number as possible.
In order to achieve this object of the invention, in an optical detector, an optical pickup and an optical information reproducing apparatus using the optical pickup, the optical information reproducing apparatus comprises: a semiconductor laser having a first laser source, a second laser source or both; a light separation element for separating a light beam emitted from the first laser source, a second laser source or both into at least three light beams; a converging optical system for applying a light beam including the three light beams to a first optical information recording medium or a second optical information recording medium different from the first optical information recording medium; an optical detector having a first light reception area disposed at a position where a light beam emitted from the first laser source and reflected from a first optical information recording medium is applied, and a second light reception area disposed at a position where a light beam emitted from the second laser source and reflected from a second optical information recording medium is applied; and a signal processing circuit for generating a focus error signal and a tracking error signal of the light spot applied to the optical information recording medium by performing a predetermined calculation of photoelectrically converted signals obtained by the optical detector and for reproducing an information signal recorded in the optical information recording medium, wherein the optical detector has the first and second light reception areas each having four divided light reception planes of a -character shape, and the signal processing circuit outputs a signal capable of generating the focus error signal by an astigmatism method by independently using the first or second light reception area or both and a signal capable of generating the tracking error signal by a differential phase detection method.
Further, in order to achieve this object of the invention, the optical detector further comprises a third light reception area and a fourth light reception area each having two divided light reception planes and disposed at a position where a light beam emitted from the first laser source and reflected from the first optical information recording medium is applied, wherein the signal processing circuit outputs a signal capable of generating the tracking error signal by a push-pull method by independently using the third or fourth light reception area or both.
Still further, in order to achieve this object of the invention, the optical detector further comprises a third light reception area and a fourth light reception area each disposed at a position where a light beam emitted from the first laser source and reflected from the first optical information recording medium is applied, wherein the signal processing circuit outputs a signal capable of generating the tracking error signal by a three-beam method by using the third and fourth light reception areas.
Still further, in order to achieve this object of the invention, the optical detector further comprises a third light reception area and a fourth light reception area each having four divided light reception planes of a -character shape and disposed at a position where a light beam emitted from the first laser source and reflected from the first optical information recording medium is applied, wherein the signal processing circuit outputs a signal capable of generating the focus error signal by the astigmatism method by independently using the third or fourth light reception area or both and a signal capable of generating the tracking error signal by the push-pull method by independently using the third or fourth light reception area or both.
Further, in order to achieve this object of the invention, the optical detector further comprises a fifth light reception area and a sixth light reception area each having two divided light reception planes and disposed at a position where a light beam emitted from the second laser source and reflected from the second optical information recording medium is applied, wherein the signal processing circuit outputs a signal capable of generating the tracking error signal by the push-pull method by independently using the fifth or sixth light reception area or both.
Still further, in order to achieve this object of the invention, the optical detector further comprises a fifth light reception area and a sixth light reception area each disposed at a position where a light beam emitted from the second laser source and reflected from the second optical information recording medium is applied, wherein the signal processing circuit outputs a signal capable of generating the tracking error signal by the three-beam method by using the fifth and sixth light reception areas.
Still further, in order to achieve this object of the invention, the optical detector further comprises a fifth light reception area and a sixth light reception area each having four divided light reception planes of a -character shape and disposed at a position where a light beam emitted from the second laser source and reflected from the second optical information recording medium is applied, wherein the signal processing circuit outputs a signal capable of generating the focus error signal by the astigmatism method by independently using the fifth or sixth light reception area or both and a signal capable of generating the tracking error signal by the push-pull method by independently using the fifth or sixth light reception area or both.
In order to achieve this object of the invention, the optical information reproducing apparatus comprises: a semiconductor laser having a first laser source, a second laser source or both; a light separation element for separating a light beam emitted from the first laser source, a second laser source or both into at least three light beams; a converging optical system for applying a light beam including the three light beams to a first or second optical information recording medium; an optical detector having first to third light reception areas disposed at positions where the three light beams emitted from a first laser source and reflected from a first optical information recording medium can be received independently, and fourth to sixth light reception areas disposed at positions where the three light beams emitted from a second laser source and reflected from a second optical information recording medium can be received independently; and a signal processing circuit for generating a focus error signal and a tracking error signal of the light spot applied to the first or second information recording medium by performing a predetermined calculation of photoelectrically converted signals obtained by the optical detector and for reproducing an information signal recorded in the first or second optical information recording medium, wherein the first and fourth light reception areas each have four divided light reception planes of a -character shape, the second, third, fifth and sixth light reception areas each have at least two divided light reception planes, and the signal processing circuit generates a focus error signal by an astigmatism method by independently using the first or fourth light reception area or both, generates a tracking error signal by a differential phase detection method by using the first or fourth light reception area or both, or generates a tracking error signal by a push-pull method by using the second, third, fifth, or sixth light reception area, or by independently using each of these areas.
In order to achieve this object of the invention, in the optical information reproducing apparatus, of a plurality of light spots applied to each of the first and second optical information recording media, at least one set of light spots has a space therebetween in a track vertical direction equal to about a half a track pitch of each of the first and second optical information recording media; and a ratio between track pitches of the first and second optical information recording media is generally equal to a ratio between wavelengths of the first and second laser sources.
The first or second laser source or both may be housed in the same package.
The signal processing circuit for generating the focus error signal and tracking error signal of a light spot applied to the optical information recording medium through a predetermined calculation of photoelectrically converted signals obtained by the optical detector, may by provided in the optical detector or optical pickup.
A method of dividing the light reception area of the optical detector into four divided light reception planes of the -character shape will be described. With this method, the light reception area of the optical detector is divided into four divided light reception planes by first and second division line crossing each other. The shape of the light reception area after this division takes a -character of kanji.
In order to achieve this object of the invention, an optical pickup or an optical information reproducing apparatus using the optical pickup comprises: a semiconductor laser having a first laser source, a second laser source or both; a light separation element for separating a light beam radiated from the first laser source, the second laser source or both into at least three light beams; a converging optical system for converging light beams including the three light beams on a first optical information recording medium or a second optical information recording medium different from the first optical information recording medium and forming independent light spots at predetermined positions of the first or second optical information recording medium; an optical detector having first and second light reception areas, the first light reception area being disposed so as to make a first position where a light beam radiated from said first laser source and reflected from the first optical information recording medium is applied, be in a light reception range, and the second light reception area being disposed so as to make a second position where a light beam radiated from said second laser source and reflected from the second optical information recording medium is applied, be in a light reception range; and a light detection optical system for guiding the light beam reflected from the first optical information recording medium or the light beam reflected from the second optical information recording medium, to a predetermined position of the optical detector.
The light detection optical system may have a function of guiding the first light beam radiated from the first semiconductor laser source and reflected from the optical information recording medium to the first light reception area disposed at the predetermined position of the optical detector and a function of guiding the second light beam reflected from the optical information recording medium to the second light reception area of the optical detector disposed at a position different from the first light reception area.
The light detection optical system may have a hologram element having a linear or curved grating groove pattern. As one example of the hologram element, the hologram element has a wavelength selectivity of not diffracting the first light beam having a predetermined wavelength and diffracting the second light beam having a wavelength different from that of the first light beam at a predetermined diffraction efficiency.
As another example of the hologram element, the hologram element has a polarization selectivity of not diffracting a light beam having a predetermined polarization direction and diffracting a light beam having a polarization direction orthogonal to the predetermined polarization direction, and is provided with a polarizer element disposed in an optical path of the first or second light beam radiated from the first or second semiconductor laser source, reflected from the optical information recording medium, and propagating toward the hologram element, the polarizer element giving a predetermined polarization direction not diffracted by the hologram element to the first light beam and giving a polarization direction diffracted by the hologram element to the second light beam.
The converging optical system may have a function of converging the first light beam radiated from the first semiconductor laser source and focussing it upon the first optical information recording medium having a predetermined substrate thickness and a function of converging the second light beam radiated from the second semiconductor laser source and focussing it upon the second optical information recording medium having a thickness different from the predetermined substrate thickness of the first optical information recording medium.
The optical pickup described above is characterized in that the first semiconductor laser source is a semiconductor laser source having a wavelength of 660 nm or shorter, the first optical information recording medium is an optical disc having a substrate thickness of about 0.6 mm, the second semiconductor laser source is a semiconductor laser source having a wavelength of 780 nm to 790 nm, and the second optical information recording medium is an optical disc having a substrate thickness of about 1.2 mm. The polarizer element is characterized in that it functions as a 5xcex/4 plate for the first light beam radiated from the first semiconductor laser source.
In the optical pickup described above, an astigmatism method is used for detecting a focus error signal from the first light beam radiated from the first semiconductor laser source and reflected from the first optical information recording medium, an astigmatism method, knife edge method or beam size method is used for detecting a focus error signal from the second light beam radiated from the second semiconductor laser source and reflected from the second optical information recording medium, a differential phase detection method or differential push-pull method (DPP method) is used for detecting a tracking error signal from the first light beam radiated from the first semiconductor laser source and reflected from the first optical information recording medium, and a push-pull method, differential push-pull method or three-spot method is used for detecting a tracking error signal from the second light beam radiated from the second semiconductor laser source and reflected from the second optical information recording medium.
For the differential push-pull method for detecting the tracking error signal of the second optical information recording medium, a three-spot diffraction grating is provided which diffracts and separates the second light beam radiated from the second semiconductor laser source into at least three light beams and sets the distance between the three light beams to about a half of the track pitch of information tracks of the second optical information recording medium along the radial direction of the second optical recording medium. The three-spot diffraction grating also diffracts and separates the second light beam radiated from the second semiconductor laser source into at least three light beams and sets the distance between the three light beams to about a half of the pitch of grooves preformed in a predetermined write-once or rewritable optical information recording medium along the radial direction of the first optical recording medium.
By adjusting the position of the light detection optical system, the positions of the light reception areas of the optical detector where the first or second light beam reflected from the optical information recording medium is applied, can be adjusted.
By adjusting the position of the light detection optical system, the positions of the light reception areas of the optical detector where the first or second light beam reflected from the optical information recording medium is applied, can be made linear.
By adjusting the position of the light detection optical system, the positions of the light reception areas of the optical detector where the first or second light beam reflected from the optical information recording medium is applied, can be made coincident.
An optical pickup or an optical information reproducing apparatus of this invention comprises: a semiconductor laser having a first laser source, a second laser source or both; a light separation element for separating a light beam radiated from the first laser source, the second laser source or both into at least three light beams; a converging optical system for converging light beams including the three light beams on a first optical information recording medium or a second optical information recording medium different from the first optical information recording medium and forming independent light spots at predetermined positions of the first or second optical information recording medium; an optical detector having first and second light reception areas, the first light reception area being disposed so as to make a first position where a light beam reflected from the first optical information recording medium is applied, be in a light reception range, and the second light reception area being disposed so as to make a second position where a light beam reflected from the second optical information recording medium is applied, be in a light reception range; and a light detection optical system for guiding the light beam reflected from the first optical information recording medium or the light beam reflected from the second optical information recording medium, to a predetermined position of the optical detector.
In the optical pickup, the light detection optical system may guide the light beam reflected from the first optical information recording medium or the light beam reflected from the second optical information recording medium to the optical detector so that a relative position between the first position where the light beam reflected from the first optical information recording medium and the second position where the light beam reflected from the second optical information recording medium is changed.
In the optical pickup, the light detection optical system may guide the light beam reflected from the first optical information recording medium or the light beam reflected from the second optical information recording medium to the optical detector so that the first light reception area disposed so as to make the first position where the light beam reflected from the first optical information recording medium is applied, be in the light reception range, is disposed at a position different from the second light reception area disposed so as to make the second position where the light beam reflected from the second optical information recording medium is applied, be in the light reception range.
In the optical pickup, the light detection optical system may guide the light beam reflected from the first optical information recording medium or the light beam reflected from the second optical information recording medium to the optical detector so that the first and second light reception areas are disposed linearly by changing a relative position between the first position where the light beam reflected from the first optical information recording medium and the second position where the light beam reflected from the second optical information recording medium.
In the optical pickup, the light detection optical system may guide the light beam reflected from the first optical information recording medium or the light beam reflected from the second optical information recording medium to the optical detector so that the first and second light reception areas are disposed at a same position by changing a relative position between the first position where the light beam reflected from the first optical information recording medium and the second position where the light beam reflected from the second optical information recording medium.
As described above, according to the present invention, the structure of the optical system of an optical pickup has a semiconductor laser with two laser sources having different wavelengths, at least one diffraction grating, and one optical detector. With this optical pickup, a focus error signal and a tracking error signal can be generated which are necessary for reproduction or record of various types of optical discs such as DVD-ROM, DVD-RAM and CD-ROM having different substrate thicknesses and groove structures. If the ratio between two laser wavelengths is approximately equal to the ratio between intervals of tracks of optical discs, the optical system requires only one diffraction grating and the wavelength characteristics and the polarization characteristics specific to optical components including the diffraction grating and half mirror are not required. Accordingly, the optical system simpler and more inexpensive than a conventional system can be realized.
If the two-wavelength multi laser is used as the light source of the optical pickup, the diffraction grating of this invention allows the light reception areas of the optical detector to be used in common for the two wavelengths. Therefore, information of a plurality type of optical discs can be reproduced by using one objective lens and one light detection optical system. The number of components can be reduced so that the optical pickup can be made more compact, simplified and more inexpensive.